In recent years, in addition to the sufficient emission intensity of light of a desired wavelength band, a long life is required for such a light source used for the above mentioned exposure apparatus for an exposure process, the image projection apparatus for digital cinema, and the spectrophotometer. Although in such a light source used in the field, arc discharge is generated between electrodes in a glass bulb that encloses mercury, rare gas (xenon gas), or both, since the electrodes are exposed to the arc discharge, they become extremely high in temperature, so that gradual evaporation thereof cannot be avoided. The problem is that the emission intensity and the light source spectrum changes gradually with the passage of lighting time, since metal evaporated from the electrodes adheres to the surface of a bulb wall so that the transmittance in the ultraviolet region of the bulb changes.
Conventionally, various measures to such a problem have been considered. For example, as shown in FIG. 7 of US Patent Application Publication No. 2007/0228300, in a laser driven light source, a laser beam from the outside is focused on gas enclosed in a quartz bulb, so as to generate plasma by exciting the enclosed gas with the laser beam to obtain a light source in which the emission intensity whose spectrum distribution according to the ingredient composition of the enclosed gas, and the luminescence center position thereof is stable.
While the laser driven light source of US Patent Application Publication No. 2007/0228300 irradiates, with the laser beam, the electric discharge gas enclosed in the quartz bulb so as to excite the electric discharge gas, thereby generating high temperature plasma, which is also irradiated with laser beam. However, all the laser beam that high temperature plasma is irradiated, is not absorbed in the high temperature plasma, and the portions of the laser beam that passes through the high temperature plasma is frequently emitted, together with light emitted from the quartz glass. It has been confirmed that the intensity of the laser beam which passes through the high temperature plasma is so high with respect to the light emitted from the quartz bulb that it cannot be ignored. Therefore, there is a possibility that peripheral devices of the laser driven light source are exposed to and destroyed by the laser beam, which passes through the high temperature plasma. However, in the laser driven light source, no measure about the laser beam that passes through the high temperature plasma, has been considered.
FIG. 13 shows a basic configuration diagram of a conventional laser driven light source, which is disclosed in Japanese Patent Application Publication No. S61-193358. A laser driven light source 130 shown in FIG. 13 is equipped with a laser oscillator 131, which oscillates (generates) a pulse-like laser beam, optical system components 132 and 133, which are suitably shaped and transmit the laser beam, an optical system component 134 for light focusing, which focuses the transmitted laser beam at a focal point in a bulb 135, the bulb 135, which encloses rare gas such as xenon gas, argon gas, or mercury vapor, etc., and a catoptric system component 136 for making the laser beam, which passes through the bulb 135, enter into the bulb once again.
In this laser driven light source 130, the laser beam from the laser oscillator 131 is suitably shaped by the optical system components 132 and 133, transmitted on the predetermined optical path, and focused by the optical system component 134 for light focusing, so as to be focused at the focal position in the bulb 135. At the focal point of the bulb 135, the enclosed gas is made into plasma by the strong electric field (high energy density) of the laser beam, and radiation of the spectrum, which includes ultraviolet rays, is made from the plasma. The laser beam, which does not contribute to the plasma generation, enters onto the catoptric system component 136, reflected thereon, and focused again at the focal point in the bulb 135.
Since there is no electrode in the bulb of the laser driven light source 130, neither the emission intensity nor the spectrum changes by evaporation or influence of sputtering, so that a life span thereof is long. In addition, since in the laser driven light source 130, the center position of the light emission is determined by the focal position of the laser beam from the outside, does not change even the bulb is replaced. and can be always maintained stably. The laser driven light source 130 is useful with respect to these aspects.